libstdc++: Rename __detail::__maybe_empty_t alias template

The key property of this alias is not that it may be an empty type, but
that the type argument may not be used. The fact it's replaced by an
empty type is just an implementation detail.  The name was also
backwards with respect to the bool argument.

This patch changes the name to better reflect its purpose.

	* include/std/ranges (__detail::__maybe_empty_t): Rename to
	__maybe_present_t.
	(__adaptor::_RangeAdaptor, join_view, split_view): Use new name.

libstdc++-v3/ChangeLog

 2020-03-02  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/ranges (__detail::__maybe_empty_t): Rename to
+	__maybe_present_t.
+	(__adaptor::_RangeAdaptor, join_view, split_view): Use new name.
+
 	* include/bits/ranges_algo.h (shift_right): Add 'typename' to
 	dependent type.
 

libstdc++-v3/include/std/ranges

@@ -1030,9 +1030,14 @@ namespace __detail
 {
   struct _Empty { };
 
-  template<bool _NonEmpty, typename _Tp>
-    using __maybe_empty_t = conditional_t<_NonEmpty, _Tp, _Empty>;
-
+  // Alias for a type that is conditionally present
+  // (and is an empty type otherwise).
+  // Data members using this alias should use [[no_unique_address]] so that
+  // they take no space when not needed.
+  template<bool _Present, typename _Tp>
+    using __maybe_present_t = conditional_t<_Present, _Tp, _Empty>;
+
+  // Alias for a type that is conditionally const.
   template<bool _Const, typename _Tp>
     using __maybe_const_t = conditional_t<_Const, const _Tp, _Tp>;
 
@@ -1065,8 +1070,8 @@ namespace views
       {
       protected:
 	[[no_unique_address]]
-	  __detail::__maybe_empty_t<!is_default_constructible_v<_Callable>,
-				    _Callable> _M_callable;
+	  __detail::__maybe_present_t<!is_default_constructible_v<_Callable>,
+				      _Callable> _M_callable;
 
       public:
 	constexpr
@@ -2211,8 +2216,9 @@ namespace views
 
       static constexpr bool _S_needs_cached_begin = !random_access_range<_Vp>;
       [[no_unique_address]]
-	__detail::__maybe_empty_t<_S_needs_cached_begin,
-				  __detail::_CachedPosition<_Vp>> _M_cached_begin;
+	__detail::__maybe_present_t<_S_needs_cached_begin,
+				    __detail::_CachedPosition<_Vp>>
+				      _M_cached_begin;
 
     public:
       drop_view() = default;
@@ -2592,8 +2598,8 @@ namespace views
 
       // XXX: _M_inner is "present only when !is_reference_v<_InnerRange>"
       [[no_unique_address]]
-	__detail::__maybe_empty_t<!is_reference_v<_InnerRange>,
-				  views::all_t<_InnerRange>> _M_inner;
+	__detail::__maybe_present_t<!is_reference_v<_InnerRange>,
+				    views::all_t<_InnerRange>> _M_inner;
 
     public:
       join_view() = default;
@@ -2728,8 +2734,8 @@ namespace views
 
 	  // XXX: _M_current is present only if "V models forward_range"
 	  [[no_unique_address]]
-	    __detail::__maybe_empty_t<forward_range<_Vp>,
-				      iterator_t<_Base>> _M_current;
+	    __detail::__maybe_present_t<forward_range<_Vp>,
+					iterator_t<_Base>> _M_current;
 
 	public:
 	  using iterator_concept = conditional_t<forward_range<_Base>,
@@ -2969,7 +2975,7 @@ namespace views
 
       // XXX: _M_current is "present only if !forward_range<V>"
       [[no_unique_address]]
-	__detail::__maybe_empty_t<!forward_range<_Vp>, iterator_t<_Vp>>
+	__detail::__maybe_present_t<!forward_range<_Vp>, iterator_t<_Vp>>
 	  _M_current;
 
 
@@ -3180,8 +3186,9 @@ namespace views
       static constexpr bool _S_needs_cached_begin
 	= !common_range<_Vp> && !random_access_range<_Vp>;
       [[no_unique_address]]
-	__detail::__maybe_empty_t<_S_needs_cached_begin,
-				  __detail::_CachedPosition<_Vp>> _M_cached_begin;
+	__detail::__maybe_present_t<_S_needs_cached_begin,
+				    __detail::_CachedPosition<_Vp>>
+				      _M_cached_begin;
 
     public:
       reverse_view() = default;